The invention relates to digital type radio broadcasting and television broadcasting systems in which digital signals"" are sent out in a number N of carrier frequencies, N ranging from some hundreds to some thousands, the spacing between neighboring carrier frequencies varying from some hundreds of hertz to some kilohertz.
As an indication, the signal that is sent out in digital radio broadcasting has a band width of 1536 Khz and attenuation of about 50 dB between the useful band and the shoulders as can be seen in the drawing of FIG. 1. In digital television broadcasting, the signal transmitted has a band width of 7.61 Mhz and attenuation which also is about 50 dB. To send out a signal of this kind, the power amplifier positioned at output of the modulator must accept a signal having a high peak factor Fc (Fc=maximum power/mean power) and must thus accept high power peaks. This results in an oversizing of the amplifier leading to high cost.
An aim of the present invention is a method and a system making it possible to control and reduce this peak factor Fc so as to reduce the size and therefore the cost of the power amplifier.
The invention relates to a method of frequency-division multiplexing in which the output signals from a frequency-division multiplexer are converted into temporal digital signals with components I and Q modulated in quadrature at the frequency f0, the signals resulting from the modulation in quadrature being applied to a peak factor amplifier Fc, wherein said method comprises the following additional operations consisting in:
(a) processing the digital signals given by the operation of modulation in quadrature or by the scaling operation according to a relationship determined to obtain other digital signals having characteristics tending to reduce their peak factor Fc, and
(b) filtering the digital signals processed by the preceding operation to attenuate the spectral components introduced by the processing of the digital signals outside the useful band.
The determined processing relationship may be obtained by a clipping of the digital signals or by a correspondence table.
Should the method be applicable to signals modulated in quadrature, the device according to the invention comprises;
a circuit for the processing of the digital signals given by the operation of modulation in quadrature according to the determined relationship, and
a band-pass filter centered on the frequency f0 of modulation in quadrature.
Should the method be applicable to the components in quadrature I and Q, the device according to the invention comprises, for each component I and Q:
a circuit for the processing of the signals given by the scaling operation, and
a low-pass filter,
the digital signals given by the low-pass filter being then modulated in quadrature at the frequency fo.